GlobalISel: Combine `op undef, x` to 0

[volkan]

Summary

Add a combine to transform op undef, x to 0. Support only G_SHL for now.

[arsenm]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

1858

The isReg check should be unnecessary

llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir

15

Also test a vector?

[volkan]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

1858

MO.getReg() may cause a crash without that check. This shouldn't a problem for this case, but I'd like to keep it as this function might be used by other combines too.

[volkan]

• Added a test case with vector type.

[volkan]

• Fixed liveins.

[paquette]

llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir

5

A testcase where the G_IMPLICIT_DEF has more than one use would be good?

[volkan]

llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir

5

Another user wouldn't have any impact on this case. Could you explain why that would be good?

[paquette]

LGTM now that I realized I just misread the code here

llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir

5

Oh, I misread the code. I thought you were replacing the G_IMPLICIT_DEF for some reason when I first read this. :P

[volkan]

https://github.com/llvm/llvm-project/commit/1242dd330d9054a57c1403f16d5487f9e3a3a92f

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

   /// Check if operand \p OpIdx is zero.
   bool matchOperandIsZero(MachineInstr &MI, unsigned OpIdx);
 
+  /// Check if operand \p OpIdx is undef.
+  bool matchOperandIsUndef(MachineInstr &MI, unsigned OpIdx);
+
   /// Erase \p MI
   bool eraseInst(MachineInstr &MI);
 

llvm/include/llvm/Target/GlobalISel/Combine.td

          [{ return Helper.matchAnyExplicitUseIsUndef(*${root}); }]),
   (apply [{ Helper.replaceInstWithConstant(*${root}, -1); }])>;
 
+def binop_left_undef_to_zero: GICombineRule<
+  (defs root:$root),
+  (match (wip_match_opcode G_SHL):$root,
+         [{ return Helper.matchOperandIsUndef(*${root}, 1); }]),
+  (apply [{ Helper.replaceInstWithConstant(*${root}, 0); }])>;
+
 // Instructions where if any source operand is undef, the instruction can be
 // replaced with undef.
 def propagate_undef_any_op: GICombineRule<

 // FIXME: These should use the custom predicate feature once it lands.
 def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
                                      undef_to_negative_one,
+                                     binop_left_undef_to_zero,
                                      propagate_undef_any_op,
                                      propagate_undef_all_ops,
                                      propagate_undef_shuffle_mask,

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

                        MRI);
 }
 
+bool CombinerHelper::matchOperandIsUndef(MachineInstr &MI, unsigned OpIdx) {
+  MachineOperand &MO = MI.getOperand(OpIdx);
+  return MO.isReg() &&
+         getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF, MO.getReg(), MRI);
+}
+
 bool CombinerHelper::replaceInstWithFConstant(MachineInstr &MI, double C) {
   assert(MI.getNumDefs() == 1 && "Expected only one def?");
   Builder.setInstr(MI);

llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -o - -mtriple=aarch64-unknown-unknown -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs %s | FileCheck %s
+---
+name:            test_combine_shl_undef_x
+body:             |
+  bb.1:
+  liveins: $w0
+    ; CHECK-LABEL: name: test_combine_shl_undef_x
+    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; CHECK: $w0 = COPY [[C]](s32)
+    %0:_(s32) = COPY $w0
+    %1:_(s32) = G_IMPLICIT_DEF
+    %2:_(s32) = G_SHL %1(s32), %0(s32)
+    $w0 = COPY %2(s32)
+...